I've had both in the duct and outside the duct.  I'm beginning to wonder if hanging the esc in the duct may actually be detrimental.</p>

Two things I'm seeing on a duct mount. One the esc could be screwing with airflow, causing a turbulence issue. And two the esc is exposed to high speed airflow, you ingest something and it hits the esc and could cause some problems.  Swallow a June bug/ beetle or other hard shelled insect and I could see an impeller exploding and blowing back into the esc.  Fan is cheap to replace compared to an esc.  </p>

My F-20 exhaust tube looked like an insect debris field after only  a couple of flights over a meadow.  Blades had all kinds of remnants on them.</p>

And I thought only gassers needed clean up after flying.</p>